The present invention relates to plastic substrates. In particular, the present invention relates to the use of polyglutarimides as substrates for electronic display applications.
Electronic displays are devices used to exhibit electronically generated graphical and textual images. There are many different types of electronic displays, and many more are being developed. Among the more familiar types of electronic displays are: liquid crystal displays ("LCDs"); cathode ray tubes ("CRTs"); arrays of light-emitting diodes ("LEDs"); plasma display panels ("PDP"); field emission displays ("FED"); and electroluminescent ("EL") displays. As electronics become more and more miniaturized, there has been an increased demand for portable devices. LCDs and EL displays are particularly useful in such portable electronic devices due to their light weight and low power consumption relative to other display technologies.
The basic LCD unit is a cell made of two transparent sheets (referred to in the art as substrates) having conductive coatings, wherein the coated sheets are sandwiched together along the edge with an adhesive cell-sealing material and filled with liquid crystal and spacers. Additional components which may be included in a liquid crystal display cell include, for example, alignment layers used to align the liquid crystal fluid, color filter coatings, active electronic devices such as thin film transistors, and polarizers. EL displays are generally composed of at least one transparent substrate having a conductive coating, and make use of electroluminescent phosphors as the image generating medium. Electronic devices made with these types of displays include other electronic components, such as drivers, which are used to generate the image on the electronic display.
Substrates for display devices have typically been made of optical quality glass or quartz because these materials meet requirements for optical clarity and flatness, and because these materials have good gas and moisture barrier properties and good thermal and chemical resistance. On the other hand, glass and quartz are brittle and subject to breakage, and they are therefore limited in usefulness in thin sheets or films less than 1 mm in thickness.
Various plastics have been proposed as replacement substrates because they are lighter weight, may be formed into thinner sheets or even films, and are more resistant to breakage. High temperature plastics (those having Tg of at least 150.degree. C.) are preferred in order to provide adequate thermal stability during the display manufacturing process. Such materials include, for example, thermoplastic resins such as polyacrylates, polysulfones, polycarbonates and polyimides.
U.S. Pat. No. 4,802,742 (Ichikawa et al) describes the use of certain plastics, such as polyether sulfone and polycarbonate, as plastic substrate materials for LCDs. Polyether sulfone has a Tg of 220.degree. C. and is therefore preferred over other plastics for use as substrate material; however, this resin has an amber color, and is therefore limited in application to thin films. Polycarbonate has less color and a higher white light transmission than polyether sulfone, but its Tg (150.degree. C.) is undesirably low, and it has relatively high stress-optic coefficient (resulting in higher birefringence).
Electronic displays made with these types of plastic substrate materials have been under development for about 20 years, but they have enjoyed only limited commercial success because all of these plastics have limitations which interfere with their performance as substrate materials. These limitations include unsatisfactory: optical quality, flatness, solvent resistance, thermal resistance, and gas barrier properties. Coatings have typically been used to improve some of these deficiencies in the plastic substrates.
EP 770 899 A1 (Paczkowski) discloses a method and composite for double sided processing of plastic substrates useful as plates in liquid crystal displays such that the composite can be separated after processing to form both sides of a liquid crystal cell. The focus of this application is on a method of simultaneously processing two pieces of plastic substrate to minimize the differences in processing conditions between the two plates which form the sides of the LCD cell. There is no discussion in this application of selection of substrate materials except with regard to UV absorption of the material and there is no discussion of coatings for the substrate other than ITO.
There remains a need for a plastic substrate material which has good optical quality, flatness, solvent resistance, thermal resistance, and gas barrier properties.